This invention relates to load tap changers and more particularly to reversing switches for load tap changers.
As is well known, a transformer converts electricity at one voltage to electricity at another voltage, either of higher or lower value. A transformer achieves this voltage conversion using a primary coil and a secondary coil, each of which are wound on a ferromagnetic core and comprise a number of turns of an electrical conductor. The primary coil is connected to a source of voltage and the secondary coil is connected to a load. Voltage present on the primary coil is induced on the secondary coil by a magnetic flux passing through the core. The voltages induced on each turn of the secondary coil are cumulative and therefore the voltage output from the secondary coil is proportional to the strength of the magnetic flux and the number of turns in the secondary coil. Since the amount of magnetic flux generated by the primary coil is proportional to the number of turns in the primary coil and the voltage produced by the secondary coil is proportional to the magnetic flux surrounding the secondary coil, the output voltage of the transformer is generally equal to the input voltage times the ratio of the number of turns in the primary coil over the number of turns in the secondary coil. Thus, by changing the ratio of primary turns to secondary turns, the ratio of input to output voltage can be changed, thereby controlling or regulating the output voltage of the transformer. This ratio can be changed by selectively connecting the load to different connection points or “taps” located at different turns of the secondary coil. A device that can make such selective connections to the taps of a secondary coil is referred to as a “tap changer”. Typically only a portion of the secondary coil is tapped.
Generally, there are two types of tap changers: on-load tap changers and de-energized or “off-load” tap changers. De-energized tap changers use circuit breakers to isolate the transformer from the voltage source and then switches the load (output) manually from one coarse adjustment tap to another. An on-load tap changer (or simply “load tap changer”) switches the load connection between taps while the transformer is connected to the voltage source. On-load tap changers typically include a plurality of selector switch contacts and, depending upon the type of load tap changer, either a selector switch with contacts that both make and break the current during tap changes or a transfer or diverter switch. The selector switch contacts correspond to a desired tap, while the diverter or transfer switch connects the desired tap to the load. In the latter, the selector switch contacts make a new tap connection before disconnecting an old tap connection, i.e., short circuits the taps. In order to avoid a high circulating current between the taps, the diverter switch temporarily places large impedance (using resistors or reactors) in series with the short-circuited taps to avoid high circulating currents between the short-circuited taps. The selector switch contacts may or may not perform any switching operations between contacts carrying current. The diverter, or transfer switch, however, does make and break connections between contacts carrying current.
In addition to the selector switch contacts and the diverter or transfer switch, a load tap changer often also includes a reversing switch, which is operable to change the polarity of the terminals of the tapped portion of the secondary winding. A change in polarity causes the voltage generated in the tapped portion of the secondary winding to either boost (increase) or buck (decrease) the voltage across the un-tapped portion of the secondary winding, depending on the position of the reversing switch. A reversing switch typically includes first and second contacts respectively connected to opposite ends of the series winding and a third contact member connected to a neutral terminal of the transformer. A movable contact assembly is operable to connect the third contact to the first contact or the second contact.
Some conventional reversing switches, particularly certain types of older ones, are susceptible to heating and coking, which can cause transformer outages and failures. Accordingly, reversing switches of certain types typically have to be replaced often during the life of a transformer. As can be appreciated, replacing a reversing switch or the entire load tap changer is expensive. Therefore, it would be desirable to provide a load tap changer with an improved reversing switch and a method of retrofitting an existing load tap changer to have an improved reversing switch. The present invention is directed toward such a load tap changers and method.